ENH: iohub KeyboardCharEvent Removed

* added duration, press_event_id, and char attributes to base keyboard
event class. Keyboard release events set the duration and press_event_id
fields if a corresponding press event was recorded. Otherwise these
fields default to '' and 0.0.

* keyboard event .char field will be used in future changes; unused now.

* populating duration and press_event_id fields is implemented and
tested on windows only at this time.

* All references to KeyboardCharEvent class removed; all usage of
EventConstants.KEYBOARD_CHAR replaced with KEYBOARD_RELEASE